1. Field of the Invention
The present invention relates to a nitride compound semiconductor light emitting device produced on a nitride compound semiconductor substrate and a method for producing the same.
2. Description of the Related Art
In the prior art, nitride compound semiconductors have been used in or studied for use in light emitting devices and high power devices, utilizing their advantageous characteristics.
For example, a nitride compound semiconductor light emitting device can technically be capable of emitting light of a wide range of wavelengths, e.g., from violet to orange, by appropriately adjusting the composition thereof. In recent years, blue light emitting diodes and green light emitting diodes have been put into practical use utilizing the advantageous characteristics of nitride compound semiconductors. As semiconductor laser devices, blue-violet semiconductor laser devices have also been developed in the art.
When producing a nitride compound semiconductor film, sapphire, SiC, spinel, Si, GaAs, GaN, or the like, may be used as a substrate. Where sapphire, for example, is used as a substrate, a GaN or AlN buffer layer is formed at a low temperature of 500xc2x0 C. to 600xc2x0 C. prior to the epitaxial growth of a GaN film. Thereafter, the substrate is heated to a high temperature of 1000xc2x0 C. to 1100xc2x0 C. and a nitride compound semiconductor film is epitaxially grown. It is known in the art that in this way, it is possible to obtain a structurally and electrically desirable crystal also having a good surface condition. It is also known in the art that where SiC is used as a substrate, it is desirable to use a thin AlN film as a buffer layer at a growth temperature at which an epitaxial growth process is performed.
However, where a substrate other than a nitride compound semiconductor substrate, e.g., a GaN substrate, is used, a large amount of defects (e.g., dislocations) may be introduced into the produced nitride compound semiconductor due to differences in thermal expansion coefficient and in lattice constant between the substrate and the nitride compound semiconductor film grown thereon. The total density of such defects may be as high as about 1xc3x97109 cmxe2x88x921 to 1xc3x97107 cmxe2x88x922. Dislocations of such a density are known to trap the carriers which control the electrical conduction of the nitride compound semiconductor substrate, thereby deteriorating the electrical characteristics of the produced film. Such dislocations are also known to shorten the operating lifetime of a laser device which uses a high level current.
In order to reduce the resulting lattice defects and to improve the electrical characteristics, various methods have been tried in the art, including a hydride vapor phase epitaxy (H-VPE), a high pressure synthesis method, a sublimation method, and the like, to form a thick film of a nitride compound semiconductor, e.g., GaN, having a thickness of about 20 xcexcm or more, which can be used as a nitride compound semiconductor thick film substrate.
By using such a nitride compound semiconductor thick film substrate, it is possible to reduce the density of defects reaching the substrate surface and to obtain a light emitting device having desirable characteristics.
However, even with such a nitride compound semiconductor thick film having a thickness over 20 xcexcm on which a nitride compound semiconductor film is epitaxially grown (hereinafter, referred to also as a xe2x80x9cnitride compound semiconductor substratexe2x80x9d), edge dislocations which extend in a direction perpendicular to the C axis are not completely eliminated and an amount of dislocations of about 1xc3x97106 cmxe2x88x922 or more still exists. It has been found that such dislocations, even though the amount thereof is reduced by an order of magnitude as compared with those resulting when using other types of substrates, adversely affect the emission intensity and the operating lifetime of a light emitting device such as a laser diode (hereinafter, referred to also as a xe2x80x9claser devicexe2x80x9d) to which a high density current is injected.
A nitride compound semiconductor substrate doped with no impurity exhibits a high electrical resistance. Such an electrical resistance has to be reduced by doping with an impurity. However, a number of problems arise when a certain amount of impurity is injected during growth of a GaN thick film by using an N-VPE method, or the like, as in the prior art. For example, when a nitride compound semiconductor substrate which has been produced by injecting a certain amount of high concentration impurity thereinto is used in a nitride compound semiconductor laser device, the threshold voltage is reduced, but the thermal current density increases on the other hand. This is due to a mutual diffusion which occurs through dislocations in the crystal between an impurity doped into the nitride compound semiconductor substrate and an impurity doped into a film which is epitaxially grown on the substrate as a part of the light emitting device structure. Thus, a current barrier is partially formed at the interface between the nitride compound semiconductor substrate and the epitaxially grown film. This gives rise to adverse influences, e.g., an increase in the driving voltage of the light emitting device and a reduction in the operating lifetime of the light emitting device.
Moreover, regarding the surface morphology of the nitride compound semiconductor substrate doped with a high concentration of an impurity, such a nitride compound semiconductor substrate has a substantial surface roughness as compared with that of nitride compound semiconductor substrates. Therefore, although a laser device produced on such a substrate has a reduced threshold voltage, the threshold current density tends to increase due to scattering of propagated light caused by the substantial surface roughness.
In order to provide a light emitting device having improved electrical characteristics and a desirable operating lifetime, it has been desired to produce a nitride compound semiconductor substrate having a substrate surface (hereinafter, referred to also as a xe2x80x9cgrowth surfacexe2x80x9d) on which a nitride compound semiconductor film is epitaxially grown with a reduced dislocation density and a desirable electrical contact between the substrate and the epitaxially grown film.
In order to solve these problems, it is important to reduce the defect density of the nitride compound semiconductor substrate and to appropriately control the electrical contact between the growth surface of the nitride compound semiconductor substrate and the epitaxially grown film on the nitride compound semiconductor substrate.
According to one aspect of this invention, there is provided a nitride compound semiconductor light emitting device, including: a nitride compound semiconductor substrate; and alight emitting device section including a nitride compound semiconductor provided on the nitride compound semiconductor substrate. The nitride compound semiconductor substrate contains a group VII element as an impurity.
In one embodiment of the invention, the nitride compound semiconductor substrate contains as its main components nitride and gallium.
Thus, a group VII element having a large ion radius is introduced into a crystal of another element having a small ion radius (e.g., nitrogen, gallium, or aluminum) which forms the nitride compound semiconductor substrate, thereby stopping the propagation of displacements to the surface of the crystal. As a result, the dislocation density in the surface of the nitride compound semiconductor substrate is reduced. The use of such a substrate increases the emission intensity and the operating lifetime of the light emitting device epitaxially grown on the nitride compound semiconductor substrate.
In one embodiment of the invention, the light emitting device section includes: a nitride compound layer of a first conductivity type; a cladding layer of the first conductivity type provided on the nitride compound layer of the first conductivity type; a light propagation layer of the first conductivity type provided on the cladding layer of the first conductivity type; a well layer provided on the light propagation layer of the first conductivity type; a carrier blocking layer of a second conductivity type provided on the well layer; a light propagation layer of the second conductivity type provided on the carrier blocking layer of the second conductivity type; a cladding layer of the second conductivity type provided on the light propagation layer of the second conductivity type; and a contact layer of the second conductivity type provided on the cladding layer of the second conductivity type.
In one embodiment of the invention, the nitride compound semiconductor substrate has a thickness of 20 xcexcm or more. Thus, the propagation of dislocations to the surface of the crystal is stopped.
As a result, the dislocation density in the surface of the nitride compound semiconductor substrate is reduced.
In one embodiment of the invention, a concentration of the group VII element contained in the nitride compound semiconductor substrate is equal to or greater than 2xc3x971014 cmxe2x88x923 and less than or equal to 2xc3x971020 cmxe2x88x923.
Thus, the effect of reducing the dislocation density is optimized, thereby improving the emission intensity and the operating lifetime of the light emitting device.
In one embodiment of the invention, the group VII element is chlorine.
Thus, it is possible to reduce the dislocations in a manner most suitable for a GaN substrate, thereby increasing the emission intensity and the operating lifetime of the light emitting device epitaxially grown on the nitride compound semiconductor substrate.
In one embodiment of the invention, the nitride compound semiconductor substrate contains an impurity for controlling electrical conduction characteristics of the nitride compound semiconductor substrate.
Thus, the chlorine doping reduces the edge dislocations in the crystal, thereby reducing the diffusion of the impurity for controlling the electrical conduction characteristics of the nitride compound semiconductor substrate into the epitaxially grown film through the edge dislocations. As a result, the voltage-current characteristics and the operating lifetime of the light emitting device epitaxially grown on the nitride compound semiconductor substrate.
In one embodiment of the invention, the impurity for controlling the electrical conduction characteristics of the nitride compound semiconductor substrate is at least one element selected from the group consisting of silicon, germanium, carbon, selenium, sulfur and oxygen. A concentration of the at least one element is equal to or greater than 1xc3x971017 cmxe2x88x923 and less than or equal to 5xc3x971020 cmxe2x88x923.
In one embodiment of the invention, the group VII element is chlorine; and a concentration of the chlorine contained in the nitride compound semiconductor substrate is equal to or greater than 1xc3x971015 cmxe2x88x923 and less than or equal to 1xc3x971020 cmxe2x88x923.
Thus, the reduction in the edge dislocations in the nitride compound semiconductor substrate which exhibits an n-type conductivity is optimized and the reduction in the impurity diffusion is also optimized. As a result, a light emitting device epitaxially grown on the nitride compound semiconductor substrate which has been optimized with such conditions has improved voltage-current characteristics and a prolonged operating lifetime.
In one embodiment of the invention, the concentration of the group VII element contained in the nitride compound semiconductor substrate in the vicinity of a surface of the nitride compound semiconductor substrate on which the light emitting device section is deposited is greater than that in other portions of the nitride compound semiconductor substrate.
Thus, as compared with a case where a high concentration of chlorine is doped across the entire area of the nitride compound semiconductor substrate, it is possible to reduce the surface roughness of the nitride compound semiconductor substrate. In addition, by the coexistence of the impurity for controlling the electrical conduction characteristics of the nitride compound semiconductor substrate, it is possible to reduce the Schottky barrier which is localized at the interface between the substrate and the epitaxially grown film, thereby further improving the voltage-current characteristics of the epitaxially grown light emitting device, reducing the threshold voltage and improving the operating lifetime.
In one embodiment of the invention, the group VII element is chlorine; and a concentration of the chlorine in a region of the nitride compound semiconductor substrate at a depth of 2 xcexcm or less from the surface of the nitride compound semiconductor substrate is greater than those in other regions of the nitride compound semiconductor substrate.
In one embodiment of the invention, the group VII element is chlorine; and a concentration of the chlorine in a region of the nitride compound semiconductor substrate at a depth of about 0.05 xcexcm from the surface of the nitride compound semiconductor substrate is equal to or greater than 1xc3x971016 cmxe2x88x923 and less than or equal to 1xc3x971020 cmxe2x88x923.
Thus, the reduction in the dislocations in the surface of the nitride compound semiconductor substrate and the reduction in the surface roughness thereof are optimized, and the voltage-current characteristics, the threshold current and the operating lifetime of the light emitting device epitaxially grown on the substrate are optimized.
According to another aspect of this invention, there is provided a method for producing a nitride compound semiconductor light emitting device by forming a light emitting device section containing a nitride compound semiconductor on a nitride compound semiconductor substrate, the method including the steps of: forming the nitride compound semiconductor substrate on a nitride compound semiconductor layer, the nitride compound semiconductor substrate containing as impurities a group VII element and an element for controlling an electrical conduction characteristics of the nitride compound semiconductor substrate; and forming the light emitting device section containing a nitride compound semiconductor on the nitride compound semiconductor substrate.
In one embodiment of the invention, the step of forming the light emitting device section includes the steps of: forming a nitride compound layer of a first conductivity type on the nitride compound semiconductor substrate; forming a cladding layer of the first conductivity type on the nitride compound layer of the first conductivity type; forming a light propagation layer of the first conductivity type on the cladding layer of the first conductivity type; forming a well layer on the light propagation layer of the first conductivity type; forming a carrier blocking layer of a second conductivity type on the well layer; forming a light propagation layer of the second conductivity type on the carrier blocking layer of the second conductivity type; forming a cladding layer of the second conductivity type on the light propagation layer of the second conductivity type; and forming a contact layer of the second conductivity type on the cladding layer of the second conductivity type.
In one embodiment of the invention, the nitride compound semiconductor substrate contains as its main components nitride and gallium.
In one embodiment of the invention, the group VII element is chlorine.
In one embodiment of the invention, the nitride compound semiconductor layer includes a striped growth suppression film.
With such a production method, it is possible to control the concentration of an impurity and to provide a concentration gradient of the impurity. Thus, the present invention solves the above-described problems in the prior art.
Thus, the invention described herein makes possible the advantage of providing a light emitting device having desirable electrical characteristics and a desirable operating lifetime.